There were plenty of game elements before 3.x and 4e. I suspect those two will get the lions share of a 5e merger game, but what elements would you like to include, even if simply optional, from some of the earlier versions of D&D?
It's in both the PHB & DMG. The players has a description of how it works for players. The DMG has it both under henchman and under combat. Be forewarned. It is quite considerable and you may not want all of it. All those loyalty and morale tables reprinted at the end of the book were for quick reference. If you want something less check out 2e.Either the end of the NPC section or towards the front of the combat section. I'm going from memory; I don't have the book here at work.